1. Field of the Invention
The present invention relates to an image exposure device, particularly an image exposure device that is disposed with a GaN based semiconductor laser, modulates a laser beam, and scan-exposes a recording material such as photosensitive material with the laser beam, and to a laser exposure device applied thereto.
2. Description of the Related Art
Image exposure devices that scan a recording material with light and expose an image thereon using a light source device that includes an edge emitting LED (a super luminescent diode (SLD) or the like) or a semiconductor laser taking as a structural material AlGaInP, AlGaAs, or InGaAsP of a GaAs substrate shape have been proposed.
In light source devices that use such materials, GaAs, which serves as the substrate with respect to an emission wavelength, is an absorbent material, and light absorbent materials such as InGaAs are also used in opposing electrodes. For this reason, light ordinarily becomes independently closed off in an emission region of a few microns in width, and there is a relatively small amount of stray light outside of striped regions due to the effect of the absorbent materials.
The development of semiconductor lasers having a short oscillation wavelength (referred to below as “short-wavelength lasers”), such as blue-violet semiconductor lasers, has advanced steadily, and the utilization of GaN based semiconductor lasers is growing near at hand. In edge emitting LEDs or semiconductor lasers using GaN material, a material such as sapphire or SiC that is transparent with respect to the emission wavelength is used for the substrate. For this reason, stray light reaching the edges of the chip is returned to a vicinity of an active region by reflection, and stray light of various patterns is produced by plural reflections.
FIG. 15 is a pattern diagram showing a GaN based semiconductor laser 90 being used for a light source in a silver halide exposure device that spot-scans using a polygon or the like. As shown in FIG. 15, laser light emitted from the GaN based semiconductor laser 90 is collimated at a spot 194 of a predetermined size by a collimator lens 192. However, stray light (so-called EL light) 198, whose emission position and direction are random, cannot be collimated at the spot 194 and forms a blurry pattern 199.
FIG. 16 illustrates the relation between drive current and light output of the spot 194 and the relation between drive current and light output of the blurry pattern 199. As shown in FIG. 16, a large amount of power is present at the blurry pattern 199 in a region of about 0.05 mW low exposure intensity that is critical in silver halide exposure methods.
In comparison to electrophotographic methods using photosensitive materials such as photosensitive drums, the blurry pattern 199 becomes a fatal defect (i.e., the photosensitive material reacts due to the stray light) in high-resolution silver halide exposure methods, which have extremely high sensitivity. For example, when a pattern (e.g., the striped pattern shown in FIG. 17A) having a line width that is roughly the same as the diameter of the spot 194 is formed with a GaN based semiconductor laser, ordinarily an image is formed in stripes as shown in FIG. 17A. However, as shown in FIG. 17B, coloration also occurs between the stripes due to the blurry pattern 199, and a remarkably degraded image whose sharpness has dropped and that is different from the expected image is formed.
Because the light amount of the blurry pattern resulting from the stray light is small, there are not many problems in area gradation such as in electrophotographic methods. However, in continuous-tone photosensitive materials such as those used in silver halide photography, a slight background leads to blurry characters and blurry images and remarkable degradation.
In exposure of silver halide methods, which are characterized by high-resolution images, it is necessary to reduce stray light that is inherent to GaN based semiconductor lasers and exerts an adverse, critical, and fatal impact on an image.
Exposure devices that expose photosensitive materials using a semiconductor laser are used for various purposes such as laser printing. In order to improve the quality of exposed images, it is effective to reduce as much as possible the spot diameter of the laser beam by reducing the wavelength itself of the laser light.
However, there is a problem in that, when a GaN based semiconductor laser is used for the light source in an exposure device, sharpness of the spot contour cannot be obtained due to the unintended stray light even if the beam diameter is reduced by operating the optical system. Particularly in silver halide methods, in which a continuous-tone high-resolution photographic image is formed by exposing a silver halide photosensitive material that is highly sensitive, lack of sharpness of the spot contour becomes a fatal problem in comparison to electrophotographic methods in which an image is structured by halftone dots.
For example, as shown in FIGS. 28A and 28B, when output light of a GaN based semiconductor laser 1 is collimated on a surface of a printing paper 3 by a lens 2, laser-oscillated laser light 4 forms a spot 5 of a predetermined size. However, a pattern 7 having a contour that lacks sharpness is formed by EL light 6 whose emission position, direction and wavelength are random. Power of about 0.1 mW is sufficient to expose a silver halide photosensitive material that is highly sensitive. However, as shown in FIG. 29, light intensity on the printing paper 3 is fairly large due to the EL light even in low exposure intensity regions where light intensity on the printing paper 3 is about 0.1 mW in the laser spot. For this reason, when a striped pattern having a line width that is equivalent to the diameter of a spot 5 shown in FIG. 30A is formed, areas between the stripes are exposed by the EL light 6 and, as shown in FIG. 30B, only images that have inferior clarity are obtainable.